The calcium-regulated activities of chromaffin cells provides a well-studied system for investigating molecular and cell-surface mediated mechanisms of neurotoxin action. The storage granules of these cells, chromaffin granules, accumulate large concentrations of catecholamines and ATP which are eventually released by exocytosis. Isolated chromaffin granules will aggregate and fuse in the presence of calcium. We have been exploring the molecular basis of these activities. Rapid freeze/fracture electron microscopic studies and fluorescent probe studies have demonstrated that membrane-associated proteins redistribute themselves as a result of aggregation and prior to fusion. Fluorescent-labelled lipid probes have been successfully inserted into chromaffin granule membranes in vitro without altering the storage properties of the particles. Resonance energy transfers studies of calcium-promoted fusion of these membranes show that, unlike artificial phospholipid vesicles, fusion runs 5-10 fold more slowly than aggregation. These results support the previous findings that substantial rearrangment of the protein and lipid components of the membrane are required for fusion to occur. This in vitro fusion is inhibited by both organic and inorganic monovalent anions and cations and is insensitive to the presence of Mg-ATP. A soluble, calcium-specific protein (synexin) isolated from chromaffin tissue or liver enhances the ability of calcium to aggregate chromaffin granule membranes. However, we have demonstrated previously that synexin has the same effect on mitochondrial membranes, microsomes and negatively charged artificial membranes. We have recently isolated a second protein (synexin II) from adrenal medulla and liver with entirely different molecular weight, protease susceptibility and peptide fragments. Synexin II also has entirely different aggregation kinetics than synexin I, showing a long lag period before a very rapid rise in aggregate size, suggesting that some activation step is required.